Closed loop current mode control of a permanent magnet synchronous motor (PMSM), a surface permanent magnet (SPM) motor, or an interior permanent magnet (IPM) motor generally requires relatively accurate measurement of the phase current in order to maintain torque ripple created by the motor within specific limits. The phase current may be measured using a precision shunt resistor and a current sense differential amplifier. However, differential amplifiers not only amplify the differential voltage, but also some amount of common-mode voltage at the inputs. Therefore, when the phase current is calculated using the output voltage of the differential amplifier, the resulting current measurement has error due to the common-mode voltage present at the inputs at the time of the measurement.
If an anti-alias filter is used at the output of the differential amplifier, the effects of the common-mode voltage will vary as a function of a pulse width modulation (PWM) cycle. It should also be noted that the amount of common-mode voltage varies from part to part, and may be different depending on a specific differential amplifier. All of these factors present challenges when attempting to compensate for the common-mode voltage in a differential amplifier.